Parasites as indicators of intra-stock variability in marine fisheries

The complex nature of population structure in many marine fishes is being revealed by new techniques (archival tags, genetic markers, otolith microchemistry). Here, parasites of Merluccius hubbsi were evaluated as indicators in that sense. This species inhabits the Argentine Sea between the 41°S and 55°S, a region characterized by a latitudinal gradient of oceanographic conditions that could influence the structure of parasite assemblages. Eight samples from the southern stock (236 hakes), and one from a northern stock were analyzed for parasites, finding 5–17 taxa. Long-lived species, mostly larval or encysted parasites, were selected as tags and analyzed by multivariate methods based on Bray-Curtis dissimilarities. Significant differences between assemblages revealed clear latitudinal patterns in their similarity. This suggests a differential effect of the environmental conditions over parasite populations harbored by hakes caught at the extremes of the distribution of the southern stock. Therefore, intra-stock differences in the parasitefauna should be considered carefully when parasite tags are used as tools for stock identification. This is because an accurate representation of population structure is necessary for providing sufficient information to achieve management objectives, particularly when studying fish populations that inhabit large or oceanographically heterogeneous regions.